1. Field of the Invention
The invention relates to a method and a device for hyperacuity-type optical detection of a substantially rectilinear contrast edge and to a system for acquiring and tracking a target comprising at least one such contrast edge.
2. Description of the Related Art
In the technical field of imaging, a solution for obtaining a very high resolution image sensor involves associating a sensor having a high density of pixels with a suitable optical system. In addition to significant bulk, this solution is very expensive.
Another solution involves moving an image sensor by displacement micro-steps, which are sub-multiples of the spacing between the pixels thereof, and reconstituting a final image on the basis of the partial images acquired during each micro-displacement. A solution of this type involves the use of a large-capacity storage memory as well as an image reconstruction algorithm which is particularly powerful and therefore expensive in time and computer resources.
In the corresponding technical field relating to detection of the horizon for automatic steering and stabilization of a ship or an aircraft, optical detection procedures have been proposed in addition to procedures employing inertial systems. The aforementioned optical detection procedures basically consist in measuring a difference in the intensity of infrared and/or visible radiation between the sky and the ground.
Other experimental systems have also been proposed. This type of system employs a CCD sensor and an image-processing algorithm, in order to extract the horizon therefrom with appropriate accuracy. However, a system of this type necessitates high luminance and a considerable processing power.
Detection by electromagnetic waves or radar waves is employed for detecting obstacles as steel cables and small-sized objects at a relatively great distance.
Radar systems with millimeter waves are capable of detecting a 6 mm diameter steel cable at a distance of about 25 m, and ultrabroadband radar systems are capable of detecting a steel cable of the same diameter at a distance of about 80 m.
In the same field a cable having a comparable diameter can be detected up to a distance limited to 6 m, however, by procedures employing a scanning laser. The corresponding scanning laser-type detectors are voluminous, bulky and heavy, weighing from 6 to 8 kg.
More recently, a detection procedure using an electronic eye subjected to micro-scanning, which rotates relative to a substantially rectilinear contrast edge was described by Stéphane Viollet and Nicolas Franceschini, Equipe Microrobotique UMR Mouvement et Perception, CNRS/Université de la Méditerrannée 31, Chemin Joseph Aiguier 13402 Marseille Cedex 20, during the fifth workshops of the microrobotic pole and first workshops of the microrobotic RTP held on 6 and 7 Nov. 2002 in Rennes, Irisa and ENS Cachan, Antenne de Bretagne, France.
The published article which resulted from these workshops and is designated A1, describes the design and production of a specific type of visual sensor known as a “neuromimetic” visual sensor, the operating laws of which are directly inspired from the living world. This sensor, known as OSCAR, for optical scanner for the control of autonomous robots, enables the angular position of a contrast having a substantially rectilinear edge to be determined by means of two spatially offset photodiodes, on which is imposed periodic rotational microscanning inspired by observations of the retina of a fly in flight.
Measurement of the relative movement of the visual environment, in other words of the substantially rectilinear edge, is carried out using an elementary movement detector (EDM) circuit, of which the use and operating law is inspired from that of fly movement detecting neurons.
For rotational scanning at variable speed, the signal delivered by the EMD circuit from the two photodiodes is a signal of which the amplitude depends on the angular position of the contrast edge relative to the average direction of the total field of vision of the sensor formed by the two photodiodes.
For a more comprehensive description of this OSCAR sensor, reference should be made to the published article A1 relating to these two workshops.
With reference to this article, it is mentioned that this OSCAR sensor allows the production, in particular, of:                much finer acuity than the angle Δφ separating the visual axes, which are the axes of maximum sensitivity of the two photodiodes, defining the total angle of vision of the sensor, such a sensor reacting to a rotation of as little as 2.5% of the angle Δφ;        a visible minimum which is much finer than this angle Δφ, a sensor of this type easily detecting a 1 cm wide black bar at a distance of 200 cm, this bar subtending an angle of only 0.28°, that is an angle representing 7.8% of this total angle of vision Δφ.        
A sensor of this type has adequate visual acuity ability to allow the integration thereof in a visuo-motor servo control loop, the static gain of the sensor also varying minimally according to the nature of the object, its contrast or its distance.